Natural gas hydrate reservoir transformation method based on recycled concrete aggregate

Abstract

The invention provides a natural gas hydrate reservoir transformation method based on recycled concrete aggregate, and belongs to the technical field of natural gas hydrate exploiting.The method comprises the steps that fracturing fluid with low viscosity is pumped into a well to form cracks, and crack forming operation is conducted; when a crack formed by the crack forming operation reaches a design value, slurry formed by mixing fracturing fluid with medium viscosity and recycled concrete aggregate with small particle size is pumped into a well; gradually increasing the viscosity of the fracturing fluid, increasing the filling amount of the recycled concrete aggregate and gradually increasing the particle size of the recycled concrete aggregate; stopping filling operation, performing fracturing fluid gel breaking reverse discharge and carbon dioxide injection replacement natural gas hydrate mining operation, adopting recycled concrete aggregate as a modification material, injecting carbon dioxide to enable the carbon dioxide to be subjected to carbonization reaction with the aggregate, promoting the decomposition of the hydrate by utilizing the heat release property of the carbonization reaction, and improving the strength and hardness of the carbonized aggregate; the support can be used as a reservoir support.

Description

Technical Field

[0001] The invention belongs to the technical field of natural gas hydrate exploitation, and in particular is a natural gas hydrate reservoir transformation method based on recycled concrete aggregate. Background Art

[0002] Natural gas hydrate is a cage-type compound formed by a hydrogen-bonded water molecule skeleton surrounding a guest natural gas molecule. It is commonly found in permafrost areas on land and shallow seabed sediments. Due to its high energy density, considerable reserves and clean combustion characteristics, it is regarded as a new energy source that can replace traditional fossil fuels.

[0003] At present, large-scale commercial exploitation of natural gas hydrates in the sea has not been achieved, mainly due to the immaturity of the exploitation technology. Conventional in-situ hydrate exploitation methods include depressurization, thermal shock, inhibitor injection and CO2 replacement. The depressurization method is to decompose natural gas hydrates by reducing the pressure of the natural gas hydrate reservoir to below the equilibrium pressure. However, the decomposition of natural gas hydrates is an endothermic process. The depressurization method cannot provide heat for the dissociation of hydrates, which easily causes the problem of well wall freezing and secondary hydrate generation, seriously reducing the gas production efficiency. The thermal shock method is to break the stable state of natural gas hydrates by raising the temperature of the natural gas hydrate reservoir to above the hydrate equilibrium temperature. However, this method not only heats the hydrates, but also more heat is absorbed by sediments, clay and bottom water, and the energy utilization efficiency is low. The inhibitor injection method is to transfer the phase equilibrium curve of natural gas hydrates to high pressure and low temperature by injecting chemical inhibitors, making the hydrates unstable in the current reservoir, but considering the high cost of chemical reagents and environmental pollution, this method only remains in the experimental stage. The CO2 replacement method utilizes the difference in phase equilibrium characteristics between carbon dioxide hydrates and natural gas hydrates. During the process of injecting carbon dioxide into the reservoir, the formation of carbon dioxide hydrates is induced to replace methane gas. Its main advantage is that carbon dioxide can be stored while being mined, but the low replacement efficiency makes it unsuitable for commercial mining. These four conventional in-situ hydrate mining methods only focus on the in-situ decomposition of hydrates, and do not consider the collection of gas after decomposition. If a low permeability reservoir is encountered, the recovery of natural gas will become a problem, and each method has its own limitations.

[0004] Inspired by unconventional energy extraction, researchers have begun to use volume fracturing to enhance hydrate extraction. Hydraulic fracturing is a conventional method for natural gas hydrate reservoir reconstruction, and the cracks formed provide an effective path for fluid migration. Not only does it have high permeability, which facilitates gas entry into the wellbore, but it also significantly increases the single-well gas production of the natural gas hydrate reservoir. Therefore, if this method is combined with conventional hydrate extraction methods, it is expected to obtain considerable gas production. However, the decomposition of hydrates will cause the loss of cohesion in the reservoir, resulting in a decrease in its shear strength. In this case, the fractures will collapse and the reservoir transformation will lose its effect.

[0005] There are still problems in the application of existing hydraulic fracturing and fracture creation technology in increasing the production of natural gas hydrates. It is necessary to construct a transformation method that can not only promote the in-situ decomposition of hydrates but also provide stable high-permeability channels. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the technical problem to be solved by the embodiments of the present invention is to provide a natural gas hydrate reservoir reconstruction method based on recycled concrete aggregate.

[0007] In order to solve the above technical problems, the present invention provides the following technical solution: a method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate, the method comprising:

[0008] Pumping low viscosity fracturing fluid into the well to form cracks and perform fracture creation operations;

[0009] When the cracks formed by the cracking operation reach the design value, a slurry mixed with medium-viscosity fracturing fluid and small-size recycled concrete aggregate is pumped into the well;

[0010] When the recycled concrete aggregate moves to the end of the main crack, the viscosity of the fracturing fluid is gradually increased, the filling amount of the recycled concrete aggregate is increased, and the particle size of the recycled concrete aggregate is gradually increased;

[0011] When the injection volume and particle size of recycled concrete aggregate reach the designed values, the filling operation is stopped and the fracturing fluid gel breaking and back discharge and carbon dioxide injection and replacement and exploitation of natural gas hydrate operations are carried out.

[0012] After mining is completed, the well is backfilled with highly permeable recycled aggregate and injected with carbon dioxide.

[0013] As a further improvement plan: it also includes:

[0014] An offshore drilling platform is set up, a well is drilled toward the seabed surface and casing is laid, a horizontal well is drilled in the middle of the reservoir, and then cementing work is completed.

[0015] As a further improvement scheme: the steps of pumping the fracturing fluid into the well to form cracks and performing the crack-making operation include:

[0016] A lower viscosity fracturing fluid is pumped into the ground at a fast flow rate to create the primary fracture;

[0017] Gradually increase the viscosity of the fracturing fluid and pump it into the ground to keep the main fracture open and continue to expand.

[0018] As a further improvement scheme: the fracturing fluid includes 0.15%-0.5% thickener, 0.05%-0.1% cross-linking agent, 0.2%-0.5% breaker, 1%-2% surfactant, 3%-5% clay stabilizer, 0.05%-0.3% low temperature activator, 0.15%-0.5% scale inhibitor, 0.05%-0.1% bactericide, 0.5%-1% hydrate inhibitor, and the rest is seawater.

[0019] As a further improvement plan: the viscosity range of the fracturing fluid used for fracture creation is 50-200 mpa·s.

[0020] As a further improvement plan: the viscosity of the fracturing fluid in the mixed slurry of the fracturing fluid and the recycled concrete aggregate is in the range of 200-500 mpa·s.

[0021] As a further improvement, the carbon dioxide injection concentration is 90 vol%-98 vol%.

[0022] Compared with the prior art, the beneficial effects of the present invention are: using recycled concrete aggregate as a transformation material, injecting carbon dioxide to cause a carbonization reaction with the aggregate, utilizing the exothermic nature of the carbonization reaction to promote the decomposition of hydrates, and increasing the strength and hardness of the aggregate after carbonization, which can serve as a support for the reservoir. The porosity of the recycled concrete aggregate also provides a seepage channel required for mining. After mining is completed, the aggregate carbonization reaction stops and no longer releases heat, and carbon dioxide continues to be introduced to generate a mixture of carbon dioxide hydrate and recycled aggregate, which not only stabilizes the reservoir but also prevents gas leakage and achieves the storage of carbon dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a reservoir transformation method for natural gas hydrate reservoir based on recycled concrete aggregate;

[0024] Figure 2 A local enlarged image of the recycled aggregate filling cracks in a natural gas hydrate reservoir reconstruction method based on recycled concrete aggregate;

[0025] Figure 3 This is a reservoir transformation effect diagram of a natural gas hydrate reservoir transformation method based on recycled concrete aggregate;

[0026] In the figure: 1. Drilling platform; 2. Natural gas storage tank; 3. Gas separator; 4. Carbon dioxide storage tank; 5. High-pressure pump; 6. Drilling equipment; 7. Flowback fluid storage tank; 8. Fracturing fluid storage tank; 9. Agitator; 10. Recycled aggregate storage tank; 11. Casing; 12. Well pipe; 13. Vertical section; 14. Inclination section; 15. Inclination stabilization section; 16. Horizontal section; 17. Secondary fracture; 18. Main fracture; 19. Shotcrete hole; 20. Sealing tool; 21. Composite bridge plug; 22. Sea surface; 23. Overlying strata; 24. Natural gas hydrate reservoir; 25. Underlying strata; 26. Recycled concrete aggregate; 27. Mixture of carbon dioxide hydrate and recycled aggregate; 28. Fracturing command center. DETAILED DESCRIPTION

[0027] The technical solution of the present application is further described in detail below in conjunction with specific implementation methods.

[0028] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application, and cannot be understood as limiting the present application.

[0029] See also Figures 1 to 3 In one embodiment, a method for transforming a natural gas hydrate reservoir based on recycled concrete aggregates comprises:

[0030] Pumping low viscosity fracturing fluid into the well to form cracks and perform fracture creation operations;

[0031] When the cracks formed by the crack making operation reach the design value, the fracturing filling work of the recycled concrete aggregate 26 can be carried out, that is, the sand carrying capacity of the fracturing fluid is used to fill the formed cracks with the recycled concrete aggregate 26. By controlling the viscosity of the fracturing fluid and the amount of the recycled concrete aggregate 26 mixed, at the initial stage of filling the aggregate, the medium-viscosity fracturing fluid in the fracturing fluid storage tank 8 and the recycled concrete aggregate 26 in the recycled aggregate storage tank 10 are first stirred by the agitator 9 to form a slurry, and then the slurry is pumped into the cracks by the high-pressure pump 5 through the well pipe 12. The recycled concrete aggregate 26 is carried by the fracturing fluid to the front edge of the main crack 18, and a part of the recycled concrete aggregate 26 with a small particle size is stuck at the front edge of the main crack 18 and no longer moves forward and forms a blockage, so that the recycled concrete aggregate 26 is sand-blocked at the front edge of the main crack 18, forming a sand-stripping belt.

[0032] When the recycled concrete aggregate 26 moves to the end of the main crack 18, the viscosity of the fracturing fluid is gradually increased, the filling amount of the recycled concrete aggregate 26 is increased, and the particle size of the recycled concrete aggregate 26 is gradually increased, so that the large-size recycled concrete aggregate 26 is fully filled in the crack, and the recycled concrete aggregate 26 is deposited, thereby increasing the amount of recycled concrete aggregate 26 in the crack and the crack width, forming a blockage, thereby enhancing the conductivity of the crack, preventing the crack from continuing to extend along the length of the crack, the filtration loss of the fracturing fluid begins to decrease, the concentration of the recycled concrete aggregate 26 begins to increase, and the recycled concrete aggregate 26 injected later is also more difficult to pass through the desanding zone. When the pumping speed of the high-pressure pump 5 remains unchanged, the pressure of the natural gas hydrate reservoir 24 begins to increase, the crack begins to expand laterally and forms a secondary crack 17.

[0033] The viscosity of the fracturing fluid in the filling stage is controlled at 200mpa·s-500mpa·s, and the concentration of the recycled concrete aggregate 26 is controlled by a density meter on the mixer 9;

[0034] When the injection amount and particle size of the recycled concrete aggregate 26 reach the designed value, the filling operation is stopped. Finally, a large amount of recycled concrete aggregate 26 of different particle sizes is pressed into the natural gas hydrate reservoir 24. A stable wide fracture channel with high conductivity can be formed inside the natural gas hydrate reservoir 24. Finally, the fracturing fluid is broken and discharged and carbon dioxide is injected to replace the natural gas hydrate. The return fluid is stored in the return fluid storage tank 7 and can continue to be used as fracturing fluid after treatment. Among them, the entire fracturing and filling operation is monitored and controlled by the fracturing command center 28, and the downhole pressure is detected in real time to command the operation at each stage.

[0035] After mining is completed, high-permeability recycled aggregate is backfilled into the well and high-concentration carbon dioxide gas is injected to form a mixture of carbon dioxide hydrate and recycled aggregate27, completing the final reinforcement of the reservoir.

[0036] Other recycled aggregates with higher permeability are backfilled in the horizontal section 16, such as recycled aggregates made of waste ceramics and waste bricks and tiles, whose porosity is 70%-80% or higher, and carbon dioxide gas is introduced to form a mixture 27 of carbon dioxide hydrate and recycled aggregate in the cracks and the horizontal section 16, thereby forming support in the reservoir and completing the final reservoir reinforcement operation. When backfilling the horizontal section, recycled aggregates made of waste ceramics or waste bricks and tiles are used instead of recycled concrete aggregates 26 to avoid the heat release of carbonization reaction between carbon dioxide and recycled aggregate, which will affect the stability of carbon dioxide hydrates already formed in the cracks.

[0037] In this embodiment, a sealing tool 20 and a grouting hole 19 are provided on the pipe body of the well pipe 12 located in the horizontal section 16, and the end of the pipe body is blocked by a composite bridge plug 21. The composite bridge plug 21 can be a drillable bridge plug or a soluble bridge plug, etc. A gas-liquid separator is also provided on the casing.

[0038] Carbon dioxide is injected into the well through the carbon dioxide storage tank 4 to displace the natural gas. The obtained gas-liquid mixture passes through the gas-liquid separator and the separated gas mixture flows into the gas separator 3 along the casing 11, and flows into the natural gas storage tank 2 and the carbon dioxide storage tank 4 respectively. The gas flow direction is as follows: Figure 1 As shown by the arrows; the carbon dioxide injection concentration is controlled at 90vol%-98vol%, and the carbon dioxide injection volume is controlled at 1000-3000m 3 / d; After the carbon dioxide is injected into the well, it penetrates into the natural gas hydrate reservoir 24 through the cracks and replaces the natural gas hydrate. At the same time, it undergoes a carbonization reaction with the filled recycled concrete aggregate 26. The reaction equation is as follows:

[0039] Ca(OH)2+CO2→CaCO3+H2O

[0040] This reaction is an exothermic reaction, which can release about 66.4KJ / mol of heat, strengthen replacement mining, and increase the replacement amount of natural gas. The carbonization reaction generates calcium carbonate on the surface of the recycled concrete aggregate 26. Subsequently, the more difficult it is for carbon dioxide to enter the interior of the aggregate, the slower the reaction will be. This can simultaneously improve the strength of the recycled concrete aggregate 26, increase its solid phase volume, and further widen the cracks to ensure its penetration efficiency. After the aggregate surface is fully covered with calcium carbonate, the carbonization reaction gradually stops, and then the pressure reduction method can be combined to continue to mine natural gas, extending the mining cycle.

[0041] The recycled concrete aggregate 26 is made of waste concrete through mechanical grinding, and its maximum particle size design value is about 4mm-5mm, the porosity is 71%-80%, the water absorption rate is 10%-15%, the pore size range is between 20μm-200μm, the cubic compressive strength is 15MPa-20MPa, the crushing index is 20%-30%, and the apparent density is 2.6g / cm 3 -3g / cm 3 , density 1.6g / cm 3 -2g / cm 3 , permeability up to 10mm 2 -20mm 2 .

[0042] Among them, the selected recycled concrete aggregate 26 is made of waste concrete through mechanical grinding; the particle size of the recycled concrete aggregate 26 can be proportioned according to the design value and construction requirements. After the recycled concrete aggregate 26 is filled into the cracks, it will not disintegrate or break again due to the closure of the cracks. It itself has high porosity, high permeability and high strength, which improves the stability of the natural gas hydrate reservoir 24 while increasing the permeability of the natural gas hydrate reservoir 24.

[0043] The present invention can form a stable wide fracture channel with high conductivity, improve the permeability of the natural gas hydrate reservoir 24, and can improve the early production efficiency of a single well by combining the carbon dioxide replacement method, and can extend the production cycle by combining the pressure reduction method to achieve the purpose of commercial production. At the same time, it effectively utilizes recycled aggregates, reduces the impact of construction solid waste on the environment, strengthens the stability of the reservoir, and realizes the storage of carbon dioxide.

[0044] See also Figures 1 to 3 In one embodiment, it also includes: setting up an offshore drilling platform 1, drilling toward the seabed surface and laying casing 11, drilling in the middle of the reservoir to form a horizontal well, and then completing cementing work.

[0045] In this embodiment, a drilling platform 1 is installed on the sea surface 22, and the drilling platform 1 is provided with a natural gas storage tank 2, a carbon dioxide storage tank 4, a high-pressure pump 5, a drilling device 6, a return fluid storage tank 7, a fracturing fluid storage tank 8, an agitator 9, and a recycled aggregate storage tank 10. The drilling device 6 is used to drill underground, and a horizontal well is formed in the natural gas hydrate reservoir 24 and a casing 11 is laid. A well pipe 12 is arranged inside the casing 11. The natural gas hydrate reservoir 24 is located between the overlying stratum 23 and the underlying stratum 25, and then the cementing work is completed; wherein the entire horizontal well structure includes a vertical section 13, a deflection section 14, a deflection stabilizing section 15 and a horizontal section 16 from top to bottom, and the position of the horizontal section 16 is controlled in the middle of the natural gas hydrate reservoir.

[0046] See also Figures 1 to 3 In one embodiment, the steps of pumping the fracturing fluid into the well to form fractures and performing the fracture-making operation include:

[0047] A relatively low viscosity fracturing fluid is pumped into the ground at a fast flow rate to create the primary fracture 18;

[0048] The viscosity of the fracturing fluid is gradually increased and pumped into the ground to keep the main fracture 18 open and continue to expand.

[0049] In this embodiment, the fracturing fluid in the fracture-making stage is called a pre-flush fluid, which can also be divided into two parts: a pre-flush fluid and a pre-flush fluid. The pre-flush fluid is pumped into the ground at a lower viscosity and a faster flow rate to produce the main fracture 18, and the viscosity is controlled at 20 MPa·s-100 MPa·s. Then, the pre-flush fluid is pumped in with a gradually increased viscosity to keep the main fracture 18 open and continue to expand, in preparation for the injection of recycled concrete aggregate 26, and its concentration is controlled at 100 MPa·s-200 MPa·s.

[0050] See also Figures 1 to 3 In one embodiment, the fracturing fluid includes 0.15%-0.5% thickener, 0.05%-0.1% cross-linking agent, 0.2%-0.5% breaker, 1%-2% surfactant, 3%-5% clay stabilizer, 0.05%-0.3% low temperature activator, 0.15%-0.5% scale inhibitor, 0.05%-0.1% bactericide, 0.5%-1% hydrate inhibitor, and the rest is seawater.

[0051] In this embodiment, when the cementing operation is completed, the fracturing fluid is prepared, and the prepared fracturing fluid is stored in the fracturing fluid storage tank 8, and is pumped into the well by the high-pressure pump 5 to form cracks. The hydraulic fracturing method is used to perform the fracture-making operation in the natural gas hydrate reservoir 24, the purpose of which is to create cracks in the natural gas hydrate reservoir 24, which is conducive to the filling of the recycled concrete aggregate 26, so as to form cracks and maintain the cracks open during the mining process.

[0052] The fracturing fluid is prepared in advance and stored in the fracturing fluid storage tank 8. After being uniformly stirred by the stirrer 9, it is pumped into the ground from the high-pressure pump 5 and sprayed out from the spray hole 19 for fracturing. The fracturing fluid includes, by mass percentage, 0.15% of a thickener (hydroxypropyl guar gum), 0.15% of a cross-linking agent (organic boron), 0.3% of a gel breaker (ammonium persulfate), 1% of a surfactant (VES), 3.5% of a clay stabilizer (KCl), 0.15% of a low-temperature activator (citric acid), 0.5% of a scale inhibitor (EDTA), 0.1% of a bactericide (mercaptobenzimidazole), 0.6% of a hydrate inhibitor (NaCl), and the rest is seawater. The amount of fracturing fluid used for seam creation depends on the fluid efficiency of the reservoir, and accounts for 4%-9% of the total designed amount of the fracturing fluid.

[0053] The fracturing fluid of the present invention is a mixture of seawater + thickener + cross-linking agent + gel breaker + clay stabilizer, etc. The fracturing fluid has the characteristics of less filtration loss, strong sand carrying capacity, low friction resistance, good stability, strong compatibility, easy backflow, and avoidance of hydrate phase change;

[0054] In this embodiment, the fracturing fluid in the fracture-making stage does not carry aggregates and is quickly pumped into the well with a relatively low viscosity to form fractures, while the fracturing fluid in the filling stage serves to transport aggregates to the ends of the fractures. The viscosity of the fracturing fluid needs to be increased to slow down the sand bank formed by the premature precipitation of aggregates during the transportation process to ensure that the aggregates can be settled at the expected location. The larger the aggregate particle size, the greater the deadweight effect, and a fracturing fluid with a higher viscosity is required for transportation.

[0055] The present invention adopts a method of fracturing and filling the natural gas hydrate reservoir 24 by continuously filling the recycled concrete aggregate 26 step by step while creating cracks. This can not only maintain the opening of the created cracks, but also facilitate the injection of recycled aggregates and increase the scope of reservoir transformation. The continuous step-by-step injection of recycled aggregates can promote the expansion of the width of the main cracks 18 and generate secondary cracks 17. The main cracks 18 and the secondary cracks 17 are interconnected, forming a complex crack network system.

[0056] The recycled concrete aggregate 26 has the characteristics of high strength, high porosity and high permeability. It can avoid disintegration or re-fracture process in the cracks. Even if the cracks are closed during the mining process, the recycled concrete aggregate 26 itself can be used as a high-permeability channel to provide conditions for the high-speed migration of natural gas. The carbon dioxide replacement method is adopted for mining. The high-concentration carbon dioxide introduced undergoes a replacement reaction with natural gas while undergoing a carbonization reaction with the aggregate. The heat released by the carbonization reaction can strengthen the carbon dioxide replacement method. After carbonization, the strength of the aggregate is further increased, the solid phase volume is increased, and the stability of the cracks is maintained.

[0057] The present invention can enhance and maintain the reservoir seepage capacity during the hydrate decomposition process, expand the mining range, and improve the gas production and mining efficiency of a single well. It can enhance the reservoir seepage capacity, and the orderly arrangement of aggregates can effectively reduce the sand production of the reservoir. It can maintain the stability of the formation during the mining process to a certain extent, expand the mining range, and improve the gas production and mining efficiency of a single well.

[0058] After mining, the present invention can generate support for cementation of recycled aggregate and carbon dioxide hydrate at the cracks, thereby maintaining the stability of the stratum and realizing the storage of carbon dioxide.

[0059] The present invention adopts the method of filling recycled aggregates step by step while creating cracks, which is beneficial to the injection of recycled aggregates, the expansion of crack width and the generation of secondary cracks 17, and can ensure that a large number of recycled aggregates with larger particle sizes fill the cracks to form stable and highly permeable crack channels; the recycled aggregates injected by the present invention have high porosity and permeability characteristics themselves, and are fully filled into the reservoir, which can not only play a supporting role and improve the stability of the reservoir, but also have a high conductivity after the hydrate decomposition causes the cracks to close, thereby improving the gas production and mining efficiency of a single well; in the process of reservoir transformation, the present invention uses a composite bridge plug 21 to perform segmented operations on the natural gas hydrate reservoir 24 in a horizontal well, which can achieve a larger range of reservoir transformation; the present invention injects recycled aggregates of different shapes and gradations to form a high-density and stable high-conductivity crack channel.

[0060] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention, and any reference numerals in the claims should not be regarded as limiting the claims involved.

[0061] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate, characterized in that: The method comprises: Pumping low viscosity fracturing fluid into the well to form cracks and perform fracture creation operations; When the cracks formed by the cracking operation reach the design value, a slurry mixed with medium-viscosity fracturing fluid and small-size recycled concrete aggregate is pumped into the well; When the recycled concrete aggregate moves to the end of the main crack, the viscosity of the fracturing fluid is gradually increased, the filling amount of the recycled concrete aggregate is increased, and the particle size of the recycled concrete aggregate is gradually increased; When the injection volume and particle size of recycled concrete aggregate reach the designed value, the filling operation is stopped, and the fracturing fluid gel breaking and reverse discharge and carbon dioxide injection and displacement extraction of natural gas hydrate operations are carried out; After mining is completed, the well is backfilled with highly permeable recycled aggregate and injected with carbon dioxide.

2. A method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: Also includes: An offshore drilling platform is set up, a well is drilled toward the seabed surface and casing is laid, a horizontal well is drilled in the middle of the reservoir, and then cementing work is completed.

3. The method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: The steps of pumping the fracturing fluid into the well to form cracks and performing the crack-making operation include: A lower viscosity fracturing fluid is pumped into the ground at a fast flow rate to create the primary fracture; Gradually increase the viscosity of the fracturing fluid and pump it into the ground to keep the main fracture open and continue to expand.

4. The method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: The fracturing fluid comprises 0.15%-0.5% of a thickener, 0.05%-0.1% of a cross-linking agent, 0.2%-0.5% of a gel breaker, 1%-2% of a surfactant, 3%-5% of a clay stabilizer, 0.05%-0.3% of a low-temperature activator, 0.15%-0.5% of a scale inhibitor, 0.05%-0.1% of a bactericide, 0.5%-1% of a hydrate inhibitor, and the remainder is seawater.

5. The method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: The viscosity of the fracturing fluid used for fracture creation ranges from 50 to 200 mpa·s.

6. The method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: The viscosity of the fracturing fluid in the mixed slurry of fracturing fluid and recycled concrete aggregate is in the range of 200-500 mpa·s.

7. The method for transforming a natural gas hydrate reservoir based on recycled concrete aggregate according to claim 1, characterized in that: The carbon dioxide injection concentration is 90 vol%-98 vol%.

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